The long-term objective of our research is to increase the benefit that hearing impaired patients receive from their cochlear implant auditory prostheses. The approach is to improve the custom-fitting strategy by which the auditory prosthesis speech processor is adjusted to meet the characteristics of the individual patient. To do this, we need to understand the root causes of performance deficits. In our preliminary studies we have shown that most subjects can discriminate the fundamental spatial and temporal features of prosthetic stimulation needed for speech recognition, but that the ability to discriminate these features depends on the parameters of stimulation, including the location of the stimulating electrodes within their electrode array and input-output function as reflected in loudness growth. Performance varies from one stimulation site to another within an individual's cochlear implant and the pattern of this variation across stimulation sites is patient specific. These results suggest two strategies for optimizing patient performance. One strategy is to create a processor map that utilizes only those stimulation sites where performance on basic perceptual tasks is best. In support of this approach, previous studies have shown that patients tolerate a reduction in the number of stimulation sites well and that there is sometimes an improvement in performance when sites where perception is poor are removed from the processor map. A second strategy is to individually optimize the stimulation parameters on a site-by-site basis. It is known for example that stimulus level can be adjusted to improve temporal acuity. Implementation of these two strategies requires a better understanding of the basic principles underlying across-site patterns of perception in cochlear implants. Specifically, the following questions must be addressed. (1) To what extent are across-site patterns of perception similar for all perceptual measures? For example, do the underlying conditions that cause poor temporal perception also affect spatial resolution? (2) What are the relative contributions of the various psychophysical acuities to speech recognition? For example, would it be better to enhance temporal acuity at the expense, if necessary, of spatial acuity? (3) Do spatial and temporal acuity show similar dependency on stimulus level? These issues will be addressed using psychophysical and speech recognition studies in implanted human subjects. The work will deepen our understanding of the mechanisms underlying across-subject variation in speech recognition performance with cochlear implants and serve as a guide for establishing and testing clinical procedures to improve performance in individual patients.